The invention relates to mechanism for driving a print ribbon for a printing machine effective for alternately winding a print ribbon applied across the print head of the machine alternately on one spool at one side in the machine and on an opposite spool at the other side of the machine.
Print ribbon driving mechanisms for alternately winding a print ribbon on opposite spools have been previously proposed which rely on the tension or resistance to further turning of a spool when unwound such as to cause a swing lever to be effective for winding the ribbon on an immediately depleted ribbon spool. An example of such a ribbon drive construction is disclosed in Nicholas Kondur, Jr., U.S. Pat. No. 4,046,246 which discloses a swingable drive arm that is alternately effective for winding a print ribbon on one or the other of a pair of ribbon spools. Another example of such a ribbon drive construction is disclosed in Terrance J. Hebron, U.S. Pat. No. 3,880,271. The Hebron structure includes a swingable feed pawl beam which has two detented positions for this purpose in which its two notches alternately rest on a drive stud. The beam thus has two principal positions in each of which it drives one of a pair of ribbon spools. The International Business Machines Technical Disclosure Bulletin publication, Vol. 21, No. 9, February 1979, pages 3687 and 3688, discloses another example of such a structure. In this structure, a longitudinally movable shifting pawl is alternately effective to drive one or the other of two ribbon spool ratchets, and the mechanism is changed from a condition driving one of the ratchets to a condition driving the other ratchet by a shifter control which is responsive to the amount of ribbon on one of the spools. The shifter control drops when the ribbon is depleted on this spool to drive a detent bar from one position to another position to cause the shifting pawl to be changed in its operation to drive one of the ratchet wheels in lieu of the other.
In Morelli, U.S. Pat. No. 3,882,989, the ribbon advance and reversal mechanism is interconnected to the existing mechanism for raising the ribbon. Accordingly, the ribbon advance occurs during each character print cycle. During each character print cycle the stretch of ribbon intermediate the spools is drawn taut as the winding spool is advanced by the associated indexing pawl and a corresponding length of ribbon is unwound from the opposite spool. This mode of operation may be quite acceptable in the environment wherein printing is effected using a type ball or character printer, as shown. However, in a wire matrix printer the print operation drives the print wire causing the ribbon to billow into contact with the media. If the ribbon is tightly constrained over the print head, the print wires may be impeded in their movements toward the media and/or during retraction which not only interferes with the print operation, but is also likely to damage both the ribbon and the print wires. It is important that any action that causes tensioning of the ribbon occur during a non-print portion of the printer operating cycle.